Large vessels, both military and commercial, are usually fitted with north-seeking gyro compasses which are expensive instruments costing at least some tens of thousands of dollars at 1981 prices. In some vessels, for backup purposes a second gyro compass is installed, this requirement being particularly important in the case of submarines in which considerable accuracy and reliability is required.
One commonly used marine gyro compass is that known as the Sperry Mark 23 Mod L-1 which includes means of providing a North-South speed correction. Nevertheless, the accuracy of this instrument could be improved and for some purposes the accuracy is too low. A typical accuracy would be with an error range of up to 11/2.degree.. Other makes of marine gyro compass are available and one of the most accurate of such compasses is accurate to within 0.2.degree. but the cost of such a compass is an order of magnitude larger, the compass including various complicated and expensive means for correcting for accelerations of the ship.
The following patent specifications may be considered to represent the general state of the art:
GB No. 1,551,309 Bodenseewerk Geratetechnik PA1 U.S. Pat. No. 3,849,636 F. Krupp Gmbh PA1 U.S. Pat. No. 4,114,437 Bodenseewerk PA1 AN; ship north acceleration PA1 ED,EDD; azimuth error and error rate PA1 EE,EED; tilt and tilt rate PA1 EES,EESD; tilt signal and tilt signal rate PA1 G; acceleration due to gravity PA1 DMP; damping term of said level detector PA1 LAT; latitude PA1 PER; period term of said level detector PA1 K; Characteristic viscosity of the level detector PA1 .PHI.G; earth angular rotation speed PA1 .PHI.V; .PHI.G as corrected by east-west speed PA1 RG; earth's radius PA1 VE,VN; ship east and north velocity